April 2019

A team at the Helmholtz-Zentrum Berlin (HZB) has succeeded in producing inorganic perovskite thin films at moderate temperatures using co-evaporation ' rendering the process of post-tempering at high temperatures unnecessary it much easier to produce thin-film solar cells.

By co-evaporation of cesium iodide and lead iodide thin layers of CsPbI3 can be produced even at moderate temperatures. An excess of cesium leads to stable perovskite phases. image: HZB

Researchers all over the world are working intensively on the development of perovskite solar cells. The focus is mainly on ones made from metal-organic hybrid perovskites, whose crystal structure is composed of inorganic elements such as lead and iodine as well as an organic molecule. Completely inorganic perovskite semiconductors such as CsPbI₃ have the same crystalline structure as hybrid perovskites, but contain an alkali metal such as caesium instead of an organic molecule. This makes them much more stable than hybrid perovskites, but usually requires an extra production step at very high temperature ' several hundred degrees Celsius.

Researchers at the California NanoSystems Institute at UCLA have discovered that caffeine improves the thermal stability of perovskite solar cells.

'Solar cells need high thermal stability since they are constantly exposed to sunlight, which warms up the devices,' said Yang, who is also a professor of materials science and engineering at the UCLA Samueli School of Engineering. 'While perovskites are an attractive option for solar cells, the materials degrade and become less stable over time. We need them to last 20 to 30 years like traditional solar cells.'

Researchers at the National Renewable Energy Laboratory (NREL) report the creation of an efficient tandem perovskite solar cell, using a new chemical formula which also improved the structural and optoelectronic properties of the solar cell.

Most of the research efforts in the field of PSCs have focused on lead-based perovskites, which have a wide bandgap. High efficiency, low bandgap perovskites would enable the fabrication of very high efficiency all-perovskite tandem solar cells where each layer absorbs only a part of the solar spectrum and is optimally configured to convert this light into electrical energy. However, low bandgap perovskites have long suffered from large energy losses and instability limiting their use in tandems.

Researchers from the Research Center for New Generation Photovoltaics (RCNPV) in Taiwan have developed solid-state perovskite solar cells which can convert indoor light to power IoT sensors.

Research Center for New Generation Photovoltaics (RCNPV) director, Wu Chun-guey, said: "Power conversion for a perovskite solar cell with area of 0.0739 square cm is 23.7%, and the efficiency decreases to 20.9% for cell area of one square cm, 17.25% for 17.277 square cm, and 11.7% for 703 square cm".

Researchers from the Graphene Flagship have managed to increase the stability of perovskite solar cells (PSCs) using hybrids of graphene and molybdenum disulphide quantum dots.

The team used molybdenum disulphide quantum dot/graphene hybrids to address PSCs' instability issue. The collaboration between research institutions and industrial partners enabled by Graphene Flagship, yielded an ink based on graphene and related materials (GRMs). Layering this over the PSCs caused them to drastically increase in stability.

A joint research team including scientists from the Chinese Academy of Scinces (CAS), Shijiazhuang Tiedao University in China and Chiao Tung University in Taiwan has developed a novel type of highly flexible and stable perovskite-based solar cell that could be used in wearable electronics.

The team stated that current PSCs are mainly made of a polymer substrate, which has been proven fragile, unstable and not adequately waterproof. The team built a new type of PSC based on an inorganic mica substrate, which could reduce the strain in the device even under large bending deformation. Mica is a mineral that separates easily into small flat transparent pieces of rock.

A collaborative project undertaken by researchers from ICIQ's Palomares and Vidal groups, the Physical Chemistry of Surfaces and Interfaces group at the Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) and IMDEA Nanociencahave has examined the interfaces in perovskite solar cells to better understand the impact that changing the materials used in such cells has on its performance.

Perovskite solar cells with different materials as HTMs

This work sheds light on the reasons behind the differences observed in perovskite solar cells' performance by comparing four different HTMs that present close chemical and physical properties.

Researchers at EPFL, Led by Wolfgang Tress, have traced the origin of apparently high and even negative capacitance values observed in perovskite solar cells. The team has found that the large perovskite capacitances are not classical capacitances in the sense of charge storage, but just appear as capacitances because of the cells' slow response time.

perovskite solar cells seem to hold great potential, with their highly efficient and low-cost; However, issues like weak long-term stability remain a challenge. Related to this are peculiar phenomena occurring in perovskite materials and devices, where very slow microscopic processes can cause a 'memory effect' of sorts.

Researchers at NTU, lead by Assoc. Prof. Wang Hong, recently demonstrated high light extraction efficiency of perovskite photonic crystals fabricated by delicate electron-beam lithography.

The perovskite photonic crystals exhibit both emission rate inhibition and light energy redistribution simultaneously. They observed 7.9-fold reduction of spontaneous emission rate with a slower decay in perovskite photonic crystals due to photonic bandgap effect (PBG).